Implantable prostheses are commonly used in medical applications. One of the more common prosthetic structures is a tubular prosthesis which may be used as a vascular graft to replace or repair damaged or diseased blood vessel. To maximize the effectiveness of such a prosthesis, it should be designed with characteristics which closely resemble that of the natural body lumen which it is repairing or replacing.
One form of a conventional tubular prosthesis specifically used for vascular grafts includes a textile tubular structure formed by weaving, knitting, braiding or any non-woven textile technique processing synthetic fibers into a tubular configuration. Tubular textile structures have the advantage of being naturally porous which allows desired tissue ingrowth and assimilation into the body. This porosity, which allows for ingrowth of surrounding tissue, must be balanced with fluid tightness so as to minimize leakage during the initial implantation stage.
Attempts to control the porosity of the graft while providing a sufficient fluid barrier have focused on increasing the thickness of the textile structure, providing a tighter stitch construction and incorporating features such as velours to the graft structure. Further, most textile grafts require the application of a biodegradable natural coating, such as collagen or gelatin in order to render the graft blood tight. While grafts formed in this manner overcome certain disadvantages inherent in attempts to balance porosity and fluid tightness, these textile prostheses may exhibit certain undesirable characteristics. These characteristics may include an undesirable increase in the thickness of the tubular structure, which makes implantation more difficult. These textile tubes may also be subject to kinking, bending, twisting or collapsing during handling. Moreover, application of a coating may render the grafts less desirable to handle from a tactility point of view, and therefore more difficult to implant. Further such grafts may have a profile not suitable for use as an endovascular device.
It is also well known to form a prosthesis, especially a tubular graft, from polymers such as polytetrafluoroethylene (PTFE). A tubular graft may be formed by stretching and expanding PTFE into a structure referred to as expanded polytetrafluoroethylene (ePTFE). Tubes formed of ePTFE exhibit certain beneficial properties as compared with textile prostheses. The expanded PTFE tube has a unique structure defined by nodes interconnected by fibrils. The node and fibril structure defines micropores which facilitate a desired degree of tissue ingrowth while remaining substantially fluid-tight. Tubes of ePTFE may be formed to be exceptionally thin and yet exhibit the requisite strength necessary to serve in the repair or replacement of a body lumen. The thinness of the ePTFE tube facilitates ease of implantation and deployment with minimal adverse impact on the body.
While exhibiting certain superior attributes, ePTFE tubes are not without certain disadvantages. Grafts formed of ePTFE tend to be relatively non-compliant as compared with textile grafts and natural vessels. Further, while exhibiting a high degree of tensile strength, ePTFE grafts are susceptible to tearing. Additionally, ePTFE grafts lack the suture retention strength of coated textile grafts. This may cause undesirable bleeding at the suture hole. Thus, the ePTFE grafts lack many of the advantageous properties of certain textile grafts.
It is also known that it is extremely difficult to join PTFE and ePTFE to other materials via adhesives or bonding agents due to its chemically inert and non-wetting character. Wetting of the surface by the adhesive is necessary to achieve adhesive bonding. Thus, heretofore, attempts to bond ePTFE to other dissimilar materials, such as textiles, have been difficult.
It is also known to use vascular grafts in conjunction with support structures. Such support structures typically come in the form of stents, which are formed of metal or polymeric materials generally formed in a tubular structure and are used to hold a vein or artery open. Stents are well known in the art and may be self-expanding or radially expandable by balloon expansion. Examples of stent/graft configurations known in the art can be seen in U.S. Pat. Nos. 5,700,285; 5,749,880; and 5,123,917, each of which are herein incorporated by reference. It is advantageous to use stent/graft configurations because the stent provides and ensures the patency of the prosthesis, while the vascular graft provides biocompatible properties in a vessel more suitable for blood to flow through.
One method for laminating layers of ePTFE is disclosed in U.S. Pat. No. 6,139,573, the contents of which are incorporated herein by reference. The lamination process is described as using a heat-shrinkable sleeve and flowable mass particulate placed over a stent having inner and outer layers of ePTFE disposed thereover. Upon application of heat, the heat-shrinkable sleeve compresses the flowable mass articulate to provide a compressive force to permit adherence of the ePTFE layers through the openings of the stent. The use of such a heat-shrinkable sleeve and flowable mass particulate, however, complicated the lamination process. Further, the heat-shrinkable sleeve or tube is typically supplied in fixed ratios relative to the diameter of the stent, such as ratios of 4:1 or 2:1. This makes control of the amount of pressure applied, especially along the length of the stent and the ePTFE layers, difficult, leading to variability of the bonding strength along such lengths.
It is apparent that conventional textile prostheses as well as ePTFE prostheses have acknowledged advantages and disadvantages. Neither of the conventional prosthetic materials exhibits fully all of the benefits desirable for use as a vascular prosthesis.
It is therefore desirable to provide an implantable prosthesis, preferably in the form of a tubular vascular prosthesis, which achieves many of the above-stated benefits without the resultant disadvantages associated therewith. It is also desirable to provide an implantable multi-layered patch which also achieves the above-stated benefits without the disadvantages of similar conventional products.
The present invention provides a composite multi-layered implantable prosthetic structure which may be used in various applications, especially vascular applications. The implantable structure of the present invention may include an ePTFE-lined textile graft, an ePTFE graft, covered with a textile covering, or a vascular patch including a textile surface and an opposed ePTFE surface. Moreover, additional ePTFE and/or textile layers may be combined with any of these embodiments.
The composite multi-layered implantable structure of the present invention includes a first layer formed of a textile material and a second layer formed of expanded polytetrafluoroethylene (ePTFE) having a porous microstructure defined by nodes interconnected by fibrils. An elastomeric bonding agent is applied to either the first or the second layer and disposed within the pores of the microstructure for securing the first layer to the second layer.
The bonding agent may be selected from a group of materials including biocompatible elastomeric materials such as urethanes, silicones, isobutylene/styrene copolymers, block polymers and combinations thereof.
The tubular composite grafts of the present invention may also be formed from appropriately layered sheets which can then be overlapped to form tubular structures. Bifurcated, tapered conical and stepped-diameter tubular structures may also be formed from the present invention.
The first layer may be formed of various textile structures including knits, weaves, stretch knits, braids, any non-woven textile processing techniques, and combinations thereof. Various biocompatible polymeric materials may be used to form the textile structures, including polyethylene terephthalate (PET), naphthalene dicarboxylate derivatives such as polyethylene naphthalate, polybutylene naphthalate, polytrimethylene naphthalate, trimethylenediol naphthalate, ePTFE, natural silk, polyethylene and polypropylene, among others. PET is a particularly desirable material for forming the textile layer.
The bonding agent may be applied in a number of different forms to either the first or the second layer. Preferably, the bonding agent is applied in solution to one surface of the ePTFE layer, preferably by spray coating. The textile layer is then placed in contact with the coated surface of the ePTFE layer. The bonding agent may also alternatively be in the form of a solid tubular structure. The bonding agent may also be applied in powder form, and may also be applied and activated by thermal and/or chemical processing well known in the art.
The present invention more specifically provides an ePTFE-lined textile graft. The lined textile graft includes a tubular textile substrate bonded using a biocompatible elastomeric material to a tubular liner of ePTFE. A coating of an elastomeric bonding agent may be applied to the surface of the ePTFE liner so that the bonding agent is present in the micropores thereof. The coated liner is then secured to the tubular textile structure via the elastomeric binding agent. The liner and textile graft can each be made very thin and still maintain the advantages of both types of materials.
The present invention further provides a textile-covered ePTFE graft. The tubular ePTFE graft structure includes micropores defined by nodes interconnected by fibrils. A coating of an elastomeric bonding agent is applied to the surface of the ePTFE tubular structure with the bonding agent being resident within the microporous structure thereof. A tubular textile structure is applied to the coated surface of the ePTFE tubular structure and secured thereto by the elastomeric bonding agent.
Additionally, the present invention provides an implantable patch which may be used to cover an incision made in a blood vessel, or otherwise support or repair a soft tissue body part, such as a vascular wall. The patch of the present invention includes an elongate ePTFE substrate being positioned as the interior surface of a vascular wall. The opposed surface is coated with a bonding agent, such that the bonding agent resides within the microporous structure of the ePTFE substrate. A planar textile substrate is positioned over the coated surface of the ePTFE substrate so as to form a composite multi-layered implantable structure.
The composite multi-layered implantable structures of the present invention are designed to take advantage of the inherent beneficial properties of the materials forming each of the layers. The textile layer provides for enhanced tissue ingrowth, high suture retention strength and longitudinal compliance for ease of implantation. The ePTFE layer provides the beneficial properties of sealing the textile layer without need for coating the textile layer with a sealant such as collagen. The sealing properties of the ePTFE layer allow the wall thickness of the textile layer to be minimized. Further, the ePTFE layer exhibits enhanced thrombo-resistance upon implantation. Moreover, the elastomeric bonding agent not only provides for an integral composite structure, but also may add further puncture-sealing characteristics to the final prosthesis.
In further aspects of the invention, the implantable structure may be used in conjunction with radially-expandable members such as stents and other structures which are capable of maintaining patency of the implantable structure in a bodily vessel. For example, a stent may be disposed over a layer of ePTFE with the stent and the layer of ePTFE being joined to the textile tubular structure via the elastomeric bonding agent or a stent may be disposed between two ePTFE layers with the outer ePTFE layer being joined to the tubular textile structure via the elastomeric bonding agent. Any stent construction known to those skilled in the art may be used, including self-expanding stents, as well as, balloon-expandable stents.
A method of forming a composite textile and ePTFE implantable device includes the steps of (a) providing an ePTFE layer having opposed surfaces comprising a microporous structure of nodes interconnected by fibrils; (b) providing a textile layer having opposed surfaces; (c) applying a coating of an elastomeric bonding agent to one of the opposed surfaces of the ePTFE layer or the textile layer; (d) providing a hollow member having an open end and an opposed closed end defining a fluid passageway therebetween and having a wall portion with at least one hole extending therethrough, the hole being in fluid communication with the fluid passageway; (e) concentrically placing the ePTFE layer and the textile layer onto the hollow member and over the at least one hole of the hollow member to provide an interior composite layer and an exterior composite layer, thereby defining a composite assembly, wherein the interior composite layer is one of the ePTFE layer or the textile layer and the exterior composite layer is the other of the ePTFE layer or the textile layer; (f) placing the hollow member with the composite assembly within a pressure chamber; (g) applying a pressure differential so that the pressure within the chamber is greater than a pressure within the fluid passageway of the hollow member; and (h) applying heat to the bonding agent to adhesively bond the textile layer and the ePTFE layer to provide a laminated composite assembly. Further, a silicone layer may be applied or placed over the textile/adhesive/ePTFE composite prior to placement in the pressure chamber. The silicone layer acts as a transfer layer through which the pressure differential is applied and does not act by itself as a force-supplying material as with the heat-shrinkable methods of the prior art.
In one aspect of the present invention, a composite vascular prosthesis formed by the methods of the present invention has a bond shear strength of at least 5.5 g/mm2 and a variation of said bond shear strength of less than about 2. In another aspect of the present invention, a composite vascular prosthesis formed by the methods of the present invention has a bond peel strength of at least 32 g/mm and a variation of said bond peel strength of less than about 4.
Various additives such as drugs, growth-factors, anti-microbial, anti-thrombogenic agents and the like may also be employed.